I have a question for the 500+ guys - how many of you shoot bare glass (i.e. with no extenders?). I was reviewing Art Morris' blog posts the other day and noticed that he almost always has an extender (usually the 2x!) on his 600mm. I realize he has a style that requires close shots, but it mirrored my experience with the 800mm. It just didn't seem long enough - or should I say it was too long for stuff that was close to mid-distance but not long enough for nesting eagles and such
500mm F4 IS II without extender 20% of the time
500mm F4 IS II with 1.4 extender 65% of the time
400mm F5.6 without extender 10% of the time
400mm F5.6 with extender 5% of the time
I've been in situations where the 500 with the 1.4 is simply too much. if I've got the time and the bird is less likely to take off such as with the Turkey Vulture head shot I'll disengage the extender and shoot the 500mmm straight away. The 400 is quite sharp about half the weight and great for longer distance hikes (entire day) though when attaching the 1.4 III you only get center AF points (5 to be exact - cross shaped) or single point. On the 500mm all AF selections are available.